1. Field of the Invention
The present invention relates to a semiconductor laser device using a group III nitride semiconductor.
2. Description of the Prior Art
In general, group III nitride semiconductors of the formula InxGayAlzN (where 0xe2x89xa6xxe2x89xa61, 0xe2x89xa6yxe2x89xa61, 0xe2x89xa6zxe2x89xa61, and x+y+z=1) have wide energy band gaps and high thermal stability, and their band gap widths can be controlled through the adjustment of their composition. For these reasons, their application is being developed in a variety of semiconductor devices such as light-emitting devices and high-temperature devices.
As light-emitting devices, light-emitting diodes (LEDs) that emit light having luminous intensity of the order of a few candelas in a wavelength range of blue to green have already been put to practical use, and laser diodes (LDs) are in the process of being developed for practical use. With respect to laser diodes, from the early stages of their development, the feasibility of using a comparatively easily available insulating substrate, such as sapphire, has been studied.
However, in a device using a sapphire substrate, crystal distortion resulting from a large lattice mismatch between the substrate and the epitaxial layer (about 14% between the sapphire C plane and the GaN crystal) and high-density dislocation defects (108 to 1010 cmxe2x88x922) introduced into the epitaxial layer have undesirable effects on the device""s characteristics such as its working life. Moreover, where sapphire is used as the substrate of a semiconductor laser device, since the substrate and the epitaxial layer have different cleavage planes, it is difficult to obtain satisfactory end surfaces when the end surfaces of the optical resonant cavity are formed by a method relying on cleavage, a common way of forming them.
Attempts have been made to avoid these problems by using as the substrate a material other than sapphire, for example SiC or the like. This, however, does not fundamentally improve the problems associated with the size and availability of the substrate, lattice mismatch, etc.
From the viewpoint of resolving the lattice mismatch between the substrate and the epitaxial layer, reducing crystal defects, and obtaining a satisfactory crystal, the inventors of the present inventions have been developing devices using as their substrate GaN, which, like their epitaxial layer, is a group III nitride semiconductor.
As a result, it is now possible to greatly improve the characteristics of nitride semiconductor laser devices. However, the use of a GaN substrate does not always result in a satisfactory nitride semiconductor laser device. Specifically, it has been found out that, in some such devices, their operation current gradually increases, or their characteristics rapidly deteriorate. Through an intensive study in search of the causes, the inventors of the present invention have found out that there are several methods of producing a GaN substrate, each producing a substrate different in structure and quality, and that the resulting differences affect the layered structure formed on the substrate, greatly influencing the characteristics of a nitride semiconductor laser device.
An object of the present invention is to provide a nitride semiconductor laser device using a group III nitride semiconductor also as a substrate wherein the structure of the device is optimized to suit the substrate actually used in order to achieve excellent operation characteristics and a long laser oscillation life.
To achieve the above object, according to one aspect of the present invention, in a semiconductor laser device comprising a substrate of a group III nitride semiconductor, a layered structure of a group III nitride semiconductor formed on the top surface of the substrate, and an electrode formed on the top surface of the layered structure, the substrate has a dislocation-concentrated region extending from the bottom surface to the top surface thereof and a low-dislocation region constituting the remaining portion thereof other than the dislocation-concentrated region, the layered structure has a stripe-shaped laser optical waveguide region located right above the low-dislocation region of the substrate, and the electrode is located right above the low-dislocation region of the substrate.
This semiconductor laser device has a substrate of a group III nitride semiconductor, in which a dislocation-concentrated region extends so as to vertically penetrate it. However, a laser optical waveguide region included in a layered structure of a group III nitride semiconductor is located not above the dislocation-concentrated region but above a low-dislocation region, i.e. the portion of the substrate other than the dislocation-concentrated region. Thus, even if the dislocation-concentrated region influences the structured layer and causes defects in a portion thereof above the dislocation-concentrated region, the laser optical waveguide region, located away from those defects, offers satisfactory characteristics.
Moreover, an electrode formed on the top surface of the layered structure is also located not above the dislocation-concentrated region but above the low-dislocation region. Thus, even if the defects in the portion above the dislocation-concentrated region reach the top surface of the layered structure and are exposed, the electrode is located away from those defects. This prevents current from flowing through the dislocation-concentrated region of the substrate and through a possibly defect-ridden portion of the layered structure above it, and thus helps alleviate the deterioration of the laser optical waveguide region resulting from an increase in the operation current.
According to another aspect of the present invention, in a semiconductor laser device comprising a substrate of a group III nitride semiconductor, a layered structure of a group III nitride semiconductor formed on the top surface of the substrate, and an electrode formed on the bottom surface of the substrate, the substrate has a dislocation-concentrated region extending from the bottom surface to the top surface thereof and a low-dislocation region constituting the remaining portion thereof other than the dislocation-concentrated region, the layered structure has a stripe-shaped laser optical waveguide region located right above the low-dislocation region of the substrate, and the electrode is located right below the low-dislocation region of the substrate.
This semiconductor laser device, too, has a dislocation-concentrated region extending in a substrate so as to vertically penetrate it. However, a laser optical waveguide region is located not above the dislocation-concentrated region but above a low-dislocation region. Thus, even if defects arise in a portion of the layered structured above the dislocation-concentrated region, the laser optical waveguide region, located away from those defects, offers satisfactory characteristics. On the bottom surface of the substrate, the lower end of the dislocation-concentrated region is exposed. However, since an electrode formed on the bottom surface of the substrate is located not below the dislocation-concentrated region but below the low-dislocation region, and is thus located away from where the dislocation-concentrated region is exposed. This prevents current from flowing through the dislocation-concentrated region, and thus helps alleviate the deterioration of the laser optical waveguide region resulting from an increase in the operation current.
According to still another aspect of the present invention, in a semiconductor laser device comprising a substrate of a group III nitride semiconductor and a layered structure of a group III nitride semiconductor formed on the top surface of the substrate, the substrate has a dislocation-concentrated region extending from the bottom surface to the top surface thereof and a low-dislocation region constituting the remaining portion thereof other than the dislocation-concentrated region, the layered structure has a stripe-shaped laser optical waveguide region located right above the low-dislocation region of the substrate, and current-shielding layers are formed, one in a portion of the bottom surface of the substrate located below the dislocation-concentrated region and another in a portion of the top surface of the layered structure located above the dislocation-concentrated region.
This semiconductor laser device, too, has a dislocation-concentrated region extending in a substrate so as to vertically penetrate it. However, a laser optical waveguide region is located not above the dislocation-concentrated region but above a low-dislocation region. Thus, even if defects arise in a portion of the layered structured above the dislocation-concentrated region, the laser optical waveguide region, located away from those defects, offers satisfactory characteristics. On the bottom surface of the substrate, the lower end of the dislocation-concentrated region is exposed. However, since a current-shielding layer is formed in this portion of the bottom surface of the substrate, even if part of an electrode formed on the bottom surface of the substrate is located in this portion, no current flows between the electrode and the dislocation-concentrated region.
Moreover, even if the defects arising above the dislocation-concentrated region reach the top surface of the layered structure and are exposed, since a current-shielding layer is formed in this portion of the top surface of the layered structure, even if part of an electrode formed on the top surface of the layered structure is located in this portion, no current flows between the electrode and the defect-ridden portion of the layered structure. This prevents current from flowing through the dislocation-concentrated region of the substrate and through a possibly defect-ridden portion of the layered structure, and thus helps alleviate the deterioration of the laser optical waveguide region resulting from an increase in the operation current.
According to a further aspect of the present invention, in a semiconductor laser device comprising a substrate of a group III nitride semiconductor and a layered structure of a group III nitride semiconductor formed on the top surface of the substrate, the substrate has a dislocation-concentrated region extending from the bottom surface to the top surface thereof and a low-dislocation region constituting the remaining portion thereof other than the dislocation-concentrated region, and the layered structure has a stripe-shaped laser optical waveguide region located right above the low-dislocation region of the substrate, and a current-shielding layer is formed inside the layered structure in a portion thereof located above the dislocation-concentrated region of the substrate.
This semiconductor laser device, too, has a dislocation-concentrated region extending in a substrate so as to vertically penetrate it. However, a laser optical waveguide region is located not above the dislocation-concentrated region but above a low-dislocation region. Thus, even if defects arise in a portion of the layered structured above the dislocation-concentrated region, the laser optical waveguide region, located away from those defects, offers satisfactory characteristics. Moreover, since a current-shielding layer is formed in a portion of the layered structured above the dislocation-concentrated region, even if defects arise in this portion, no current flows therethrough. This prevents current from flowing through the dislocation-concentrated region of the substrate and through a possibly defect-ridden portion of the layered structure, and thus helps alleviate the deterioration of the laser optical waveguide region resulting from an increase in the operation current.
The dislocation-concentrated region, as seen from above, may be shaped like a stripe substantially parallel to the laser optical waveguide region of the layered structure. Giving the dislocation-concentrated region such a shape makes it easy to form the laser optical waveguide region, and also makes it easy to form the electrodes and the current-shielding layer.
The current-shielding layer or current-shielding layers may be made of a dielectric containing at least one of SiO2, SiN, SiO, ZnO, PbO, TiO2, ZrO2, CeO2, HfO2, Al2O3, Bi2O3, Cr2O3, In2O3, Nd2O3, Sb2O3, Ta2O5, Y2O3, AlF3, BaF2, CeF2, Caf2, MgF2, NdF3, PbF2, SrF2, ZnS, and ZnSe.
It is preferable that the current-shielding layer or current-shielding layers have a thickness equal to or greater than 1 nm and smaller than or equal to 1 xcexcm. This helps shield current without fail, and helps prevent mechanical defects such as cracking and exfoliation.
It is preferable that the current-shielding layer or current-shielding layers have a width equal to or greater than 5 xcexcm and smaller than or equal to 300 xcexcm. This helps shield without fail the current that tends to flow through the dislocation-concentration region and through a possibly defect-ridden portion of the layered structure while preventing the current-shielding layer or current-shielding layers from hampering the flow of current to be fed to the laser optical waveguide region.